If somebody feels this or that way is a matter of their psyche (feelings/emotions) including their self-confidence (or lack thereof) and not a physical act. And penis size is not even one of my considerations.
i won't feel emasculated or less of a man just because another man is enabled to provide my Woman with sexual pleasure and orgasms. i can do that too. i am not competing with the other guy because a husband or otherwise primary partner serves a different function than a sidekick. Sex with the husband is better in some aspects of it while sex with a stranger or regular friend with benefits is better in other aspects of it. You can't compare apples with oranges.
One motivating factor, perhaps the most important one, for my decision of sharing the Woman i love is that i want to grant her the opportunity of living her life by my side to the fullest and being as happy and satisfied as can be. A man for her on the side who were a lousy lover would not be of any advantage in order to reach that goal. And i don't need such in order to up my ego because i am not lacking there anyway.
i hope i was able to explain in an understandable manner. If not, feel free to ask me further questions anytime.